Hydrazine-Assisted, Low-Temperature Aerosol Pyrolysis Method to Synthesize γ-Fe₂O₃

Oxides of iron in different crystalline forms such as hematite, maghemite, and magnetite have been used in number of industrial and biomedical applications in recent years. These materials are often synthesized in multiple-step processes in which simultaneous control of particle size, shape, morphology, and crystallinity is difficult. A single-step, low-temperature spray pyrolysis of iron precursors with hydrazine in a furnace aerosol reactor (FuAR) is described. To help select viable precursors and guide selection of operating conditions for the aerosol process, hydrazine derivatives of iron precursors are prepared in batch and analyzed with TGA, FTIR, and XRD. These results were also used to identify the chemical formula of hydrazinated precursors and elucidate the reaction pathway for the precursors such as ferrous acetate, ferric nitrate, and ferrous oxalate that are suitable for single-step aerosol reactor synthesis. These iron precursors are aerosolized and reacted in situ with hydrazine vapor to demonstrate the single-step, low-temperature production of metarstable γ-Fe₂O₃. Structurally similar cubic intermediates (FeO, Fe₃O₄, γ-FeOOH·H₂O) are the key decomposition intermediates observed during the synthesis of γ-Fe₂O₃. It is shown that hydrazinated iron precursors decompose at a lower temperature compared to the same unhydrazinated precursors to produce pure γ-Fe₂O₃. The localized exothermal reaction of hydrazine released from the hydrazinated precursor with oxygen along with a controlled supply of energy assists in the production of the metastable γ-Fe₂O₃. Hydrazine oxidation releases moisture and nitrogen as reaction byproduct, which further enhances the stability of γ-Fe₂O₃.